Federal aviation safety regulations require aircraft to provide evacuation and other safety provisions for passengers. These include evacuation slide, life rafts, life vests, and other safety devices. For example, inflatable escape slides and life rafts are generally built from an assembly of inflatable tubular structures that form airbeams that are sealed to one another. A balance between strength and weight must be reached during the design process. The material must be appropriately flame resistant, have low friction to allow passenger sliding, have sufficient strength to withstand high inflation forces, resist tearing and abrasion, but also be light enough so as to not unduly add to aircraft weight.
Evacuation slides, rafts, life vests, or other emergency flotation devices are inflatables typically formed from woven base substrates. A woven base substrate is typically coated and/or laminated in order to give it the desired air holding characteristics. As background, woven fabric constructions are characterized by two sets of yarns: warp and weft yarns. Warp yarns are raised and lowered to make “sheds,” and weft yarns are passed through these sheds, perpendicular to the warp yarns (and may be referred to as fill or pick yarns). The woven substrates give strength and rigidity for inflatable tubular structures.
However, such a woven architecture introduces a “crimp effect” or undulations in the yarns as they pass alternately over and under one another during the weaving process. Yarn “crimp” is the waviness of warp yarns and weft yarn interlacing together to produce the fabric construction. It is affected by yarn count, fabric structure, and weaving tensions related to the strength of the textile fabric. If a load is applied on a woven fabric and the yarns are not crimped, the full load will be faced in tension at complete strength. However, if the yarns are bent or crimped, the initial load will be consumed in straightening the bent yarns, and then take upload. The use of woven construction can thus lead to “low strength materials.” The crimp effect can also influence fiber volume fraction, which eventually leads to compromised mechanical performance of this fabric. Specific features that may be compromised are tensile and compressive properties.
When woven fabrics are used for inflatable structures, and particularly when used to create inflatable tubular structures that are cylindrical in shape, the inflatable structure experiences load in three directions. First, there is a circumferential stress or hoop stress, which is a normal stress in the tangential direction. Second, there is an axial stress, a normal stress parallel to the axis of cylindrical symmetry. Third, there is a radial stress, a stress in directions coplanar with, but perpendicular to, the symmetry axis. Thin sections of inflatable fabric will generally have negligible radial stress. However, the hoop stress is generally two times the axial/longitudinal stress. The practical effect of this is that an inflatable tube, such as an evacuation slide or life raft tube, experiences two times more stress in the hoop direction compared to the length direction. Examples of these two stresses and how they are experienced along a tubular structure is illustrated by FIG. 3. However, current woven substrates used for inflatable tubular structures are constructed with yarns having the same or similar strength in both the hoop and the axial/longitudinal direction. This can add to unnecessary and undesirable weight to the overall structure.
Another challenge that can be presented by the use of woven substrates occurs during coating of the substrates. Empty voids 10 in between the yarns 12 can create high and low points that pose challenges during the coating process. This is generally referred to as the peak-valley effect, and is illustrated by FIG. 1. In order to make these fabrics “air holding” or having “gas barrier” properties, multiple layers of coatings 14 are put on top of one another until the desirable thicknesses achieved. A final top coating 16 may be applied for rendering air holding features. These multiple layers of coatings 14 are undesirable, as it increases weight and cost of such fabrics. Improvements to fabrics used for inflatables are thus desirable.
Nonwoven fabric is a material made from fibers that are bonded together by chemical, mechanical, heat or solvent treatment. The term is generally used in the textile manufacturing industry to refer to fabric-like materials that are neither woven nor knitted. The use of nonwoven materials has generally been limited to the medical industry (for surgical gowns and drapes), the filter industry (for various types of filtration, including coffee and tea bags, vacuum bags, and so forth), the geotextile industry (for foundation stabilizers, erosion control materials, sand and landfill liners), as other miscellaneous industries (such as for carpet backing, for diapers or feminine hygiene products, cleaning wipes, for marine sails, for parachutes, for backpacks or as batting in quilts or comforters). Nonwoven materials have not, however, been used in connection with inflatable life-saving devices as described herein. Use of nonwoven materials for these uses presents unique challenges that the present inventors have solved.